A poster is any piece of printed paper designed to be attached to a wall or vertical surface. Typically posters include both textual and graphic elements, although a poster may be either wholly graphical or wholly text. Posters are designed to be both eye-catching and informative. Posters may be used for many purposes. They are a frequent tool of advertisers (particularly of events, musicians and films), propagandists, protestors and other groups trying to communicate a message. Posters are also used for reproductions of artwork, particularly famous works, and are generally low-cost compared to original artwork.

how ever, that doesn't mean you should dip your lobster in tacky mayonnaise

The scene is about composition. The poster must fit in with the scene. You must be consistent. Remember, real life does not apply to games; what makes sense in real life rarely does in a game. The poster stands out because it does not look like it should be there or in that style within the context of the scene.

Therefore, the argument of 'purpose' is irrelevant.

I mean, come on. The scene has dominant whites, greys, yellows and greens. That poster is mainly reds and oranges. It attracts attention for no reason, and it shouldn't.

Yeah you should make a variant of that brick wall without the clouds. Just the green hills and a clear, blue sky. Use it on the center wall below the 2nd floor so you don't get those cut in half clouds. Looks awkward.

Very cool idea though. You should fill it with TONS of secret passages and whatnot, just to make it more fun to explore.